Temperatures in a parked car, even with the windows cracked, can heat up to over 120 degrees Fahrenheit within 15 minutes when the ambient temperature outside is 80 degrees.
On average 37 children per year die from vehicular heatstroke in the US, mostly attributed to either being unintentionally or intentionally left in the vehicle by an adult, or playing in unattended vehicle. Children are not the only ones at risk from being left in vehicles, Older adults and persons with chronic medical conditions are particular susceptible to extreme temperatures both heat related illnesses (heat cramps, heat exhaustion, heat syncope, or heatstroke) as well as hyperthermia. Heat related deaths and injuries to pets are also major issues, and while there is no formal tracking for pet heat-related deaths from being left in vehicles, it clearly happens too often.
While temperature related deaths are a major issues it is not the only risk when children, adults or pets are left in unattended vehicles, carbon monoxide poisoning also claims hundreds of lives a year from people sitting in running cars with faulty exhaust systems or in closed in spaces.
More recently, alarms have become available that notify a person, typically the vehicle owner, when someone has been left in a parked vehicle. However, such notifications are often ineffective because that person typically already knows that fact and, thus, may ignore any notification(s) to that effect.
Thus, there is an ongoing technological problem with protecting vehicle occupants that may be incapable of doing so themselves.